Disc brake

ABSTRACT

A disc brake having an actuation mechanism, a disc brake rotor, first and second brake pad mounting structures, and first and second brake pads. The first and second mounting structures may have formations that affect fitting of the first and second pads.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/925,241, filed Jun. 24, 2013, now U.S. Pat. No. 9,664,242, thedisclosure of which is hereby incorporated in its entirety by referenceherein.

TECHNICAL FIELD

The present invention relates to a disc brake. More particularly, thepresent invention relates to a disc brake incorporating an arrangementfor ensuring that brake pads are always fitted in their correctorientation, as well as to a brake pad having a suitable formation foruse with such a brake.

BACKGROUND

It is known from EP0752541 (ArvinMeritor, Inc.) to provide formations ona pair of brake pads and corresponding features in abutment surfaces ofa brake carrier to inhibit the brake being fully assembled with the padsorientated with the backplate thereof facing the brake disc. Incorrectlyfitting brake pads in this manner is highly undesirable since thebraking effectiveness of the brake is dramatically diminished and damageto the pad and brake disc may also occur.

In this patent, the two pads are identical and are provided with aformation that has rotational symmetry about an axis extendingperpendicular to the rotational axis of the brake disc, so that althoughthe pads cannot be fitted with the backplate facing the disc, the padsare interchangeable in an inboard and outboard sense.

In most applications such an arrangement is entirely satisfactory.However, the inventors have recognised that in some circumstancesfurther measures may be required to ensure that in an “abuse” situationa user cannot forcibly refit the pad retainer. This situation may arisebecause the vertical height difference between the stepped pad abutmentsrequired to lift an incorrectly orientated pad is limited by thestrength requirements of the brake carrier. A suitable step height inthe pad assembly and corresponding carrier abutment 62, 63 for anoptimised carrier and to prevent the incorrect installation can have amarked reduction in usable pad volume.

EP1473481 (BPW) discloses alternative pad foolproofing arrangements. Inone embodiment, lugs having rotational, but not mirror symmetry areprovided in a non-machined portion of the pad windows, proximate themachined surfaces. This location is undesirable as it is a high stressarea of the carrier, and may result in uneven wear of the pad, as one ofthe two pistons exerts the same braking force, but over a lower padarea.

In another embodiment, the lugs have mirror symmetry and are configuredonly to extend into a corresponding cut-out in the metal backplate, butnot the friction material. In both embodiments, a part-worn pad can befitted in the incorrect window facing the wrong way.

In service, part worn pads may be removed and refitted, e.g. to inspectthe pads for signs of cracking and glazing, so the applicant recognisesthat a real risk that impaired brake function may occur if such pads arewrongly fitted.

The present invention seeks to overcome or at least mitigate theproblems of the prior art.

SUMMARY

A first aspect of the present invention provides a disc brakecomprising: an actuation mechanism; a void for accommodating a discbrake rotor to be arranged to rotate about an axis; first and secondbrake pad mounting structures to be located, in use, adjacent opposinginboard and outboard faces respectively of a brake rotor to be braked;first and second brake pads located by the first and second mountingstructures respectively such that upon application of the actuationmechanism the pads clamp the rotor and brake torque is reacted by thepad mounting structure; wherein to prevent or inhibit fitting of thefirst and second pads, individually or in combination with frictionmaterial facing away from the rotor the first and second mountingstructures comprise first and second formations on radially inner facesthereof, which first and second formations extend radially outwardstherefrom and do not have mirror symmetry about a plane defined by thecentre of the rotor; wherein the formations are remote from abutmentsurfaces for supporting the first and second pad in a radial direction,and wherein a spacing axially between the rotor when in a fully worncondition, and formations is such that a fully worn pad having acomplementary formation therein is only capable of being successfullyfitted in with its remaining friction material facing the rotor.

By locating the formations away from the abutment surfaces, they are ina low stress location.

Preferably, the spacing axially between a fully worn rotor and theformations is such that an excessively worn pad having a complementaryformation therein is only capable of being successfully fitted in withits remaining friction material facing the rotor.

The formations may be positioned so as to have rotational symmetry aboutan axis extending radially from a central position between the mountingstructures.

The formation adjacent the rotor may be machined.

A face of the formation or protrusion disposed adjacent the rotor mayhave a clearance or spacing axially to the rotor when the brake rotor isunworn of less than 4 mm, preferably less than 3 mm, more preferablyless than 2 mm.

The actuation mechanism may comprise first and second pistons and theformations are positioned so as not to be contacted by the pistons inuse.

The formations are preferably closer to a centre of the mountingstructure than an end of the mounting structure.

Advantageously, the formations are proximate the centre of the mountingstructure, preferably wherein the formations have a centre and thecentre of the protrusions is within 20 mm of the centre of the mountingstructure.

More preferably, a portion of the formation meets the centre of themounting structure.

At least one of the inboard and outboard mounting structures may definea beam and the formation may extend axially towards the rotor beyond thebeam.

Preferably, the outboard mounting structure defines a beam, which beambows outboard at the centre thereof.

At least the inboard mounting structure may be provided on a fixedcarrier portion of the brake. Preferably, both the inboard and outboardmounting structures are provided on a fixed carrier portion of thebrake.

The first and second pads may further comprise substantially identicalstructures for attachment of pad springs.

The disc brake may further comprise an aperture for the fitting andremoval of the pads in a radial direction; preferably wherein theaperture permits the fitting and removal of the pads to be achieved withthe brake rotor being in place.

The pads are preferably configured to be retained radially in themounting structures by a pad retainer, preferably wherein the padretainer is prevented from being secured over the brake pads if at leastone of the pads is incorrectly orientated.

A second aspect of the present invention provides a brake pad forfitment into a disc brake, the brake pad comprising: friction materialmounted on a substantially rigid, generally planar backplate; thebackplate defining a central axis extending generally parallel to adirection of insertion and removal of the backplate into the disc brake;the pad having a recess in the backplate and in the friction material inradially inner faces thereof the recess being arranged to accommodate acorresponding protrusion in the disc brake to ensure the fitting of thepad in the correct orientation in the disc brake; and wherein the centreof the recess is offset from the central axis of the friction lining,and a portion of the recess reaches the central axis.

The friction material preferably comprises a channel extendingtherethrough in substantial alignment with the central axis.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a perspective view of an air-actuated disc brake according toan embodiment of the present invention;

FIG. 2 is a partial cross-sectional view through a caliper, inboardbrake pad and rotor of the brake of FIG. 1;

FIG. 3 is an isometric view of a carrier of the brake of FIG. 1;

FIG. 4 is an end view of the carrier of FIG. 3 looking outboard alongaxis A-A;

FIG. 5 is a detail view of the carrier of FIGS. 3 and 4;

FIG. 6 is a cross-sectional view in the plane 6-6 of FIG. 4, with brakepads in place;

FIG. 7 is an isometric view in the plane 6-6 of FIG. 4, with brake padsin place;

FIG. 8 is a detail end view looking outboard of the carrier of FIG. 3along axis A-A with an outboard brake pad only in place;

FIG. 9 is an end view of a brake pad according to an embodiment of thepresent invention;

FIG. 10 is a schematic cross-sectional view through the brake on aradial centreline of FIG. 1 with the outboard brake pad incorrectlyorientated; and

FIGS. 11A to 11F are detail views of portions of brake pads according tofurther embodiments of the present invention, incorporating variousalternative recess shapes.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

With reference to FIGS. 1, 2 and 3, a brake 1 according to an embodimentof the present invention comprises a carrier 3 fixed to an axle orsteering knuckle (not shown) in use, and has first and second brake padmounting structures in the form of “windows” or radial openings 60, 61for receiving identical inboard and outboard brake pads 2. The carrier 3straddles a brake rotor 4 mounted for rotation about an axis A-A with awheel to be braked (not shown). The outboard direction is indicated byOB and inboard by IB. Arrow R indicates a radial axis extending throughthe centre of the rotor 4 parallel to a direction of removal of brakepads 2. Arrows C indicate a circumferential direction normal to bothaxis A-A and axis R.

The brake further comprises a caliper 10 slidably mounted on the carrierfor movement along axis A-A. The caliper 10 also straddles the rotor 4and applies a clamp load to apply the brake and slow rotation of thewheel.

A known force transmission device 5 of the brake 1 is indicatedgenerally at 5 in FIG. 2 and is of the type used in the presentapplicant's ELSA/EX range and is described to illustrate the generalfunction of air disc brakes of this type. The device 5 is mounted withina chamber 9 of a housing 7 of a disc brake caliper 10. The housing 7 isadapted to mount a conventional air 50 or other power actuator—e.g. anelectromechanical actuator (not shown) on an external face thereof. Anactuating lever (not shown) may perform an angular reciprocal swingingmovement under the action of a thrust member of the power actuator, thelever being integral or attached to a rotary actuating member 13(commonly referred to as an operating shaft) which is rotatablysupported within the caliper. The member 13 is recessed to houserespective cylindrical rollers 15, 16, the axes of which are offset fromthe rotary axis of the actuating member 13 and form an eccentricactuating arrangement. The rollers 15 and 16 bear against respectivethrust assemblies, shown as adjustable piston assemblies, and indicatedgenerally at 17 and 18 of the force transmission device 5.

Rotation of the lever and its connected member 13 causes actuatingthrust to be applied via piston assemblies 17 and 18 parallel to axisA-A to an inboard directly actuated brake pad 2 and, by reaction via thecaliper 10, to an indirectly actuated outboard brake pad (see FIG. 7).The brake pads are mounted so as to face respective sides of the brakerotor 4. The carrier 3 restrains the brake pads from circumferentialmovement in direction C and radial inward movement. Radial movementoutward along axis R is restricted by pad springs 30 and a pad retainer31.

An adjuster assembly is indicated generally at 19 and may be of anyappropriate conventional type needing no detailed description. Theadjuster responds to excessive movement of the brake pad 2 (e.g. due towear of friction material 40 in use) and produces resultant rotation ofan adjuster shaft 21 via gear 52 which in turn rotates a pair of innertappets 22 and 23 of the adjustable piston assemblies 17 and 18.

When force is applied by the brake actuator from the right in FIG. 2,the entire piston assembly slides along bore 24 a to transmit thebraking force to the brake pad 2 via piston head 26. To adjust thelength of the piston assembly 17, inner tappet 22 is rotated so as tocause relative axial movement between the inner tappet and the pistonshaft 25 by virtue of the action of their mating threads. The outerpiston has a non-circular profile to prevent rotation of the pistonshaft 25 relative to the housing 24, thereby ensuring that rotation ofinner tappet 22 results in actual lengthening of the piston assembly 17.Other force transmission devices are known, but operate using similarprinciples, and the present invention remains applicable thereto.Certain known force transmission devices use a single piston assembly,rather than two.

With reference to FIGS. 7 and 9, brake pads 2 according to an embodimentof the present invention will now be discussed. In this embodiment, eachbrake pad 2 comprises a substantially planar cast iron backplate 42 of agenerally rectangular shape and having a typical thickness of around8-10 mm. In other embodiments stamped steel backplates having a typicalthickness of around 6-10 mm may instead be used. Backplates of bothtypes have suitable pad spring holding formations 32 for fixing padsprings 30 thereto. In alternative embodiments (not shown), the padspring may be fixed to the pad retainer instead.

One face of each pad 2 has friction material 40 fixed thereto using asuitable mechanical keying arrangement. The friction material typicallyhas a thickness of around 15-25 mm initially, and the pad is typicallyconsidered fully worn and in need of replacement once 2 mm of materialremains (although this minimum threshold cannot always be guaranteed tobe followed in the field).

In this embodiment a radially extending channel 44, extends through aportion of the depth friction material 40 at the centre of the frictionmaterial. Such channels are typically provided to inhibit unwantedcracking of the friction material occurring, caused by temperature orload stresses in use. In other embodiments pads 2 with no such channels,or two or more channels in other locations, may be provided.

With reference to FIG. 3, as the friction lining of the brake pad wearsand the adjuster assembly extends the piston assemblies to maintain asuitable running clearance (typically 0.6-1.1 mm between each pad 2 andthe rotor 4) the inboard pad 2 moves outboard along radial inboardabutment surfaces 62 and circumferential inboard abutment surfaces 72.Assuming equal friction material 40 wear on the inboard and outboardpads 2, and inboard and outboard faces of the rotor 4, the outboard pad2 moves inboard on its radial 63 and circumferential 73 abutmentsurfaces, as the inboard pad moves out outboard on its radial 62 andcircumferential 72 abutment, with the caliper tappets extending tocompensate for the sum of all wear (including rotor).

In addition the brake rotor 4 wears, albeit at a slower rate, and may beconsidered fully worn once 3 mm is lost from each face.

As the piston assemblies 17, 18 extend, the piston heads move over theinboard brake pad window 60. In addition, wear causes the caliper 10 tomove inboard with respect to the carrier and clearance in a radialdirection R between an outboard portion of the caliper and an outboardcarrier beam 66 is required.

Turning to FIGS. 4-6 in particular, the construction of the carrier 3 isgenerally of a similar format to known carriers, but with some importantdifferences in accordance with the present invention. In this embodimentthe carrier is cast as a single iron component. In other embodiments, itmay be formed from two or more components, one or more of which may beforged.

An inboard carrier beam 64 extends between the circumferential abutments72. In a circumferential direction C the beam is substantially straight,but arches radially outwards at its centre. The cross-section of theinboard beam is relatively small in this embodiment as the majority ofthe drag load from the inboard brake pad 2 is taken by the axle mountingbracket/steering knuckle (not shown) to which the carrier is secured.The cross-section reduces in area towards the centre of the beam 64.

By contrast, in this embodiment, the outboard carrier beam 66 bowsoutboard on axis A-A and radially outwards, and has a thickercross-section. The cross-section is a dog-leg profile, with a longerthinner leg extending along axis A-A and a thicker, shorter legextending radially outwards parallel axis R. Greater strength isrequired by the outboard beam to withstand the drag loads generated bythe outboard brake pad 2, and the outboard bowed shape and dog-legprofile has been found to provide an optimised strength to weight ratio.

The radial abutment surfaces 62, 63 are located proximate thecircumferential surfaces 72, 73, but spaced therefrom. The radial andcircumferential abutment surfaces 62, 72, 63, 73 are cast withadditional material and subsequently machined to provide suitably smoothand accurately dimensioned surfaces. In addition, the surfaces may betreated (e.g. induction hardened) for additional durability.

In this embodiment, faces 62′ and 63′ of the abutment surfaces that facethe rotor (i.e. are generally parallel to axis R) are also machined inorder to obtain a close enough tolerance to enable there to be a nominalclearance of approx. 1.5 mm between an unworn rotor and the faces 62′and 63′ either side of the rotor.

As such, even if the rotor is in a fully worn condition (3 mm lost fromeach face), a 6 mm steel backplate is used, and the pad is worn until nofriction lining remains, a 1.5 mm overlap exists between the pad andabutment surfaces, so the pad 2 is not ejected through the gap betweenrotor 4 and carrier. Of course, if it could be ensured that only thickercast backplates 42 are used, a greater clearance—e.g. 3 mm may beprovided, and in this instance there may be no need to machine thefacing surfaces, since this allows for the greater tolerances inherentwith cast components.

With reference to FIGS. 5, 7 and 8, in order to “foolproof” brake pad 2fitment, so the pads 2 can only be fitted radially inwardly into inboardand outboard windows 60 and 61 in the correct orientation during anassembly or maintenance operation, and the brake then fully assembled bysecuring the pads in place with the pad retainer in both unworn and wornconditions, the pads 2 comprise a recess 75 in a radially inner edgethat extends through both the friction material 40 and backplate 42. Theinboard and outboard carrier beams 64 and 66 include complementaryformations in the form of protrusions 68 and 70.

The inboard protrusion 68 is cast integrally with the beam 64 and inthis embodiment is a trapezoid with rounded corners and is ofsubstantially constant cross-section in the A-A axis. The protrusionextends outboard beyond the beam and an outboard face 68′ is machined tobe substantially co-planar with abutment surface faces 62′.Advantageously, these faces 62′ and 68′ may be machined by the same toolin the same operation. As such face 68′ has in this embodiment a nominal1.5 mm clearance to an unworn rotor.

In this embodiment the protrusion 68 is located proximate the centre ofthe beam 64 (where a plane defined by axes R and A intersects the beamat 90°), but is offset slightly to the right as viewed in FIG. 8 (byapproximately 7.2 mm to the centre of the protrusion in thisembodiment). Indeed, in this embodiment, the base of the protrusionmeets the beam at the centre thereof.

The outboard protrusion 70 is of substantially identical cross-sectionto the inboard protrusion 68 and extends inboard from outboard beam 66and terminates in the same plane as faces 63′ of the outboard radialabutment surfaces 63 (i.e. to also have a nominal 1.5 mm clearance fromthe outboard face of rotor 4), having been machined in a similaroperation. As the outboard beam 66 bows outboard, the distance theprotrusion 70 is cantilevered inboard from the beam along axis A-A isgreater than for protrusion 68. Further, the length parallel axis A-Afor which the protrusion 70 is above the level of the outboard beam 66is greater than the equivalent for protrusion 68, in order to accountfor the additional travel of the outboard pad 2 with respect to thecarrier 3. In this embodiment, the outboard protrusion length is 15 mm,and the inboard 13.6 mm.

The outboard protrusion 70 is also proximate the centre of beam 66, butis offset to the left by the same amount inboard protrusion 68 is offsetto the right (7.2 mm).

In other words, the protrusions 68 and 70 have rotational symmetry aboutaxis R, but do not have mirror symmetry about a plane defined by R anddirection C.

In this embodiment, the height differential between the radial abutmentsurfaces 62 and 63 and the tops of the protrusions is 20.9 mm. Theheight of the protrusions 68, 70 is typically limited by the clearancebetween the outboard beam 66 and the caliper because the caliper maymove over the beam when the pads 2 become worn.

The brake pads are identical, and the recess 75 therein is essentiallythe same shape as the protrusions 68 and 70, but is larger in both widthand height to provide sufficient clearance that in normal usage theprotrusions do not contact the pads 2 and therefore interfere withnormal operation thereof. In this embodiment, the clearance is around3.5 mm in order to account for clearance circumferentially of the pads 2within windows 60 and 61, tolerances in the backplates 42 andprotrusions 68, 70, and heat expansion etc.

As the friction material 40 is set back from the perimeter of thebackplate 42 by approx. 0.5 mm, the recess in the friction material isslightly larger than in the backplate.

As the recess 75 is somewhat larger than the protrusions 68, 70, it canbe seen that in this embodiment it crosses the centre of the pad (i.e.crosses a plane defined by axes R and A). This has a number of benefitsfor the pad of the present invention. Firstly, the impact of the recesson the swept pad area is minimised, as a portion of the area removedfalls within the channel 44.

Further, it avoids there being a relatively thin and therefore weakportion of friction material being left between channel 44 and therecess 75, which could be broken off in service, reducing the swept padarea. Additionally, the protrusions are located in a relatively lowstress area of the carrier 3.

Theoretically, it is possible to move the recess 75 further away fromthe centre of the pad 2. However, as illustrated in FIG. 9, the scopefor doing so is limited by the position of the piston heads 26. Giventhe heads extend over the inboard beam when the pads are worn, therecesses cannot be located within the dotted area, or will be hit by theheads. As such, the applicant has recognised this near central location,as opposed to a location proximate the radial pad abutments to beadvantageous for a number of functional reasons.

FIG. 10 shows the effectiveness of the foolproofing, even with fullyworn brake pads 2 (2 mm of friction material remaining) and a fully wornrotor 4 (3 mm wear to each face). As illustrated the inboard pad 2 hasbeen correctly fitted with friction material facing the rotor 4 as theprotrusion 68 locates within recess 75. However, the outboard pad hasbeen placed into the outboard window 61 with backplate 42 facing therotor. If successfully fitted in this orientation, this incorrectfitting would significantly impair the ability of brake to stop thevehicle to which it is fitted.

However, as can be seen the protrusion 70 does not locate within therecess, and there remains sufficient overlap between the backplate 42and the protrusion for the protrusion to prevent the backplate 42 frombeing able to seat on the radial abutment surfaces 63. In this radiallyelevated position of the pad 2 of around 9 mm, the pad retainer 31,which hooks into a recess on the housing 7, is tilted outwardly to theextent that even with the pad springs 30 depressed, a pad retainer bolt34 is unable to engage with a complementary threaded bore 36 in thecaliper 10. As a result re-assembly is prevented and the brake fitter isalerted to their error and therefore able to correct it.

FIGS. 11A to 11F illustrate various alternative shapes of recess to beused with complementary projections on the inboard and outboard beams.

Recesses 75 a and 75 b FIGS. 11A and 11B are similar to recess 75, butthe angles of the trapezoid are unequal. Recesses 75 c and 75 d arenon-symmetrical curves, recess 75 e is a shallower curve that extendspartially into the friction material on the other side of channel 44,and recess 75 f is triangular and also crosses the channel 44. It willbe appreciated that numerous other recess shapes may be utilised, andthat through careful selection thereof, they may be used not only forfoolproofing, but also to ensure that pads of a particular specificationfor one brake model are not useable on other models for which they arenot intended.

It will be appreciated that the dimensions referred to above are usedfor illustrative purposes, and will vary dependent upon the size andgeometry of the brake to which the present invention is applied. Theterm “fully worn pad” is intended to refer to a pad which has reachedits minimum design limit for the amount of friction material remainingthereon—i.e. in the specific embodiment above, 2 mm of frictionmaterial. The term “excessively worn pad” is intended to mean a pad thathas less than the minimum design limit of friction materialremaining—e.g. 1 mm or 0.5 mm.

Numerous benefits of the present invention are achieved with brake padsthat do not utilise a central channel, and with recesses locatedelsewhere in the space between the two piston heads.

Although the invention has been described above with reference to one ormore preferred embodiments, it will be appreciated that various changesor modifications may be made without departing from the scope of theinvention as defined in the appended claims. For example, similarprinciples may be applied to arrangements that have neither rotationalnor mirror symmetry—e.g. to prevent pads being fitted in the wrongorientation as well as the wrong location. An example of this may bewhere pads inboard and outboard have different strengths or otherdissimilar properties, and it would be detrimental to brake operationfor the inboard pad to be fitted outboard and/or vice versa.

The faces of the protrusions that are adjacent the rotor faces may havea greater clearance if used only with thicker cast backplate brakepads—e.g. around 3 mm. In this embodiment, machining of the faces maynot be required.

The present invention remains applicable for the inboard pad and windowat least for brakes in which the outboard pad is mounted solely by thecaliper, and only the inboard pad is located circumferentially andradially by the carrier.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A disc brake comprising: an actuation mechanism;a brake rotor arranged to rotate about a rotational axis; inboard andoutboard brake pad mounting structures to be located, in use, adjacentopposing inboard and outboard faces, respectively, of the brake rotor;and first and second brake pads located by the inboard and outboardbrake pad mounting structures, respectively, wherein upon application ofthe actuation mechanism the first and second brake pads clamp the brakerotor and brake torque is reacted by the inboard and outboard brake padmounting structures, wherein the inboard and outboard brake pad mountingstructures comprise inboard and outboard abutment surfaces,respectively, that support the first and second brake pads in a radialdirection; wherein to prevent or inhibit fitting of the first and secondbrake pads, individually or in combination with friction material facingaway from the brake rotor, the inboard and outboard brake pad mountingstructures include first and second protrusions, respectively, onradially inner faces thereof, wherein the first protrusion protrudesoutboard and protrudes radially outward from the inboard brake padmounting structure, and the second protrusion protrudes inboard andprotrudes radially outward from the outboard brake pad mountingstructure, and wherein the first and second protrusions do not havemirror symmetry about a plane defined by a center of the brake rotor andnormal to the rotational axis of the brake rotor, and wherein the firstand second protrusions are remote from the inboard and outboard abutmentsurfaces, respectively; wherein the first brake pad in a fully worncondition is successfully fittable on the inboard brake pad mountingstructure with its friction material facing the brake rotor when thefirst protrusion is received in a first complimentary recess in thefriction material of the first brake pad; and wherein there is a spacingaxially between the brake rotor and an outboard face of the firstprotrusion when the brake rotor is in a fully worn condition, andwherein the first brake pad in the fully worn condition is notinsertable between the first protrusion and the brake rotor in the fullyworn condition.
 2. The disc brake of claim 1 wherein the spacing axiallybetween a fully worn brake rotor and the outboard face of the firstprotrusion is such that an excessively worn brake pad having acomplementary recess therein is only capable of being successfullyfitted in with its remaining friction material facing the brake rotor.3. The disc brake of claim 1 wherein there is a spacing axially betweenthe brake rotor and an inboard face of the second protrusion when thebrake rotor is in a fully worn condition, and wherein the second brakepad in the fully worn condition is not insertable between the secondprotrusion and the brake rotor in the fully worn condition.
 4. The discbrake of claim 3 wherein the spacing axially between a fully worn brakerotor and the inboard face of the second protrusion is such that anexcessively worn brake pad having a complementary recess therein is onlycapable of being successfully fitted in with its remaining frictionmaterial facing the brake rotor.
 5. The disc brake of claim 1 whereinthe first and second protrusions are positioned so as to have rotationalsymmetry about an axis extending radially from a central positionbetween the inboard and outboard brake pad mounting structures.
 6. Thedisc brake of claim 1 wherein the inboard abutment surfaces each have anoutboard face, and wherein the outboard face of the first protrusionthat is disposed adjacent to the brake rotor is machined so as to besubstantially co-planar with the outboard face of the inboard abutmentsurfaces.
 7. The disc brake of claim 1 wherein the second protrusion hasan inboard face and the outboard abutment surfaces each have an inboardface, and wherein the inboard face of the second protrusion that isdisposed adjacent to the brake rotor is machined so as to besubstantially co-planar with the inboard face of the outboard abutmentsurfaces.
 8. The disc brake of claim 1 wherein the second protrusion hasan inboard face, and wherein the outboard face of the first protrusionthat is disposed adjacent to the brake rotor or the inboard face of thesecond protrusion that is disposed adjacent to the brake rotor has aspacing axially to the brake rotor when unworn of less than 4 mm.
 9. Thedisc brake of claim 1 wherein the brake rotor is fully worn once it haslost 3 mm of material from each face.
 10. The disc brake of claim 1wherein each brake pad comprises a substantially planar backplate andfriction material mounted onto the backplate, and wherein the firstbrake pad is fully worn once a 2 mm thickness of friction material ofthe first brake pad remains on the backplate, and the second brake padis fully worn once a 2 mm thickness of friction material of the secondbrake pad remains on the backplate.
 11. The disc brake of claim 1wherein the actuation mechanism comprises first and second pistons, andthe first and second protrusions are positioned so as not to becontacted by the first and second pistons, in use.
 12. The disc brake ofclaim 1 wherein each brake pad comprises a substantially planarbackplate and friction material mounted onto the backplate, wherein athickness of the backplate along an axial axis of the disc brake isgreater than an axial spacing between the brake rotor and the outboardface of the first protrusion, when the brake rotor is in the fully worncondition.
 13. The disc brake of claim 1 wherein the inboard brake padmounting structure comprises an elongate beam having two opposing endsthereof and defining a center point therebetween, wherein the outboardbrake pad mounting structure comprises an elongate beam having twoopposing ends thereof and defining a center point therebetween, furtherwherein the first and second protrusions are closer to a center of theinboard and outboard brake pad mounting structures, respectively, thanan end of the inboard and outboard brake pad mounting structures. 14.The disc brake of claim 13 wherein the first and second protrusions areproximate the center of the inboard and outboard brake pad mountingstructures, respectively.
 15. The disc brake of claim 14 wherein thefirst and second protrusions have a center, and wherein the center ofthe first and second protrusions is within 20 mm of the center of theinboard and outboard brake pad mounting structures, respectively. 16.The disc brake of claim 14 wherein a portion of each of the first andsecond protrusions meets the center of the inboard and outboard brakepad mounting structures, respectively.
 17. The disc brake of claim 1wherein at least one of the inboard and outboard brake pad mountingstructures defines a beam, and the first or second protrusions extendaxially towards the brake rotor beyond the beam.
 18. The disc brake ofclaim 1 wherein the outboard brake pad mounting structure defines abeam, and wherein the beam bows outboard at a center thereof.
 19. Thedisc brake of claim 1 wherein the first and second brake pads are eachconfigured to be retained radially in the inboard and outboard brake padmounting structures, respectively, by a pad retainer, wherein the padretainer is prevented from being secured over the first and second brakepads when at least one of the first and second brake pads is incorrectlyorientated.
 20. A disc brake comprising: an actuation mechanism; a brakerotor arranged to rotate about an axis; first and second brake padmounting structures to be located, in use, adjacent opposing inboard andoutboard faces, respectively, of the brake rotor; and first and secondbrake pads located by the first and second brake pad mountingstructures, respectively, wherein upon application of the actuationmechanism the first and second brake pads clamp the brake rotor andbrake torque is reacted by the first and second brake pad mountingstructures; wherein to prevent or inhibit fitting of the first andsecond brake pads, individually or in combination with friction materialfacing away from the brake rotor, the first and second brake padmounting structures include first and second protrusions on radiallyinner faces thereof, which first and second protrusions extend radiallyoutwards from the radially inner faces and the first and secondprotrusions do not have mirror symmetry about a plane defined by acenter of the brake rotor and normal to the axis; wherein the first andsecond protrusions are remote from abutment surfaces that support thefirst and second brake pads in a radial direction; wherein the firstbrake pad in a fully worn condition is successfully fittable on thefirst brake pad mounting structure with its friction material facing thebrake rotor when the first protrusion is received in a firstcomplimentary recess in the friction material of the first brake pad;and wherein there is a spacing axially between the brake rotor and thefirst protrusion when the brake rotor is in a fully worn condition andwherein the first brake pad in the fully worn condition is notinsertable between the first protrusion and the brake rotor in the fullyworn condition.